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Surface-enhance

Cite this: Anal. Methods, 2017, 9, 4338

Received 5th July 2017

DOI: 10.1039/c7ay90097j

rsc.li/methods

4338 | Anal. Methods, 2017, 9, 4338–4

d Raman spectroscopy (SERS) incultural heritage

Analytical Methods Committee AMCTB No. 80 (Background paper)

Surface-enhanced Raman spectroscopy (SERS) has been increasingly

used in the last decade for the identification of organic colourants in

works of art. ‘Surface-enhancement’ is the giant increase in Raman

scattering intensity experienced by organic molecules adsorbed on

atomically rough metallic surfaces. In the case of organic colourants,

which are in general strongly luminescent under laser excitation and

therefore cannot usually be identified by conventional Raman

microscopy (see AMCTB no. 67), SERS yields a strong and clearly

resolved spectrum, allowing for easy identification. Unlike ordinary

Raman spectroscopy, SERS often requires sampling, yet the sample

size is considerably smaller (from 20 to 100 micrometres) than that

required for high-performance liquid chromatography (HPLC), the

benchmark technique for organic colourants analysis. For this reason,

SERS has seen substantial use in the analysis of works of art such as

prints, drawings, paintings, and polychrome sculpture. This technical

brief focuses on the practical aspects of SERS and its application to the

analysis of cultural heritage material, as well as on its current

limitations.

Introduction

Surface-enhanced Raman spectroscopy (SERS) is a relativelynew technique, rst applied to the study of cultural heritagematerials in 1987, when Guineau and Guichard identied aliz-arin in an 8th century textile dyed with madder.1 It took,however, almost twenty years (and the widespread adoption ofRaman spectroscopy in museums) for other researchers tofollow. In the last decade a substantial amount of work on SERS-based identication of organic colourants in microscopicsamples of dyed bres, paints, and glazes has been carried out.New substrates and separation techniques have been

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developed, extraction methods have been improved, minimallyinvasive methods have been proposed, and database libraries ofspectra have been assembled.

The enormous enhancement (as high as 1012) of Ramanscattering intensity experienced by molecules adsorbed onatomically rough metallic surfaces (usually made of nano-particles of the so-called coinage metals, i.e., silver, copper orgold) results in increased sensitivity for the identication ofminor and trace components. At the same time, adsorption onmetallic surfaces generally quenches uorescence, thusenabling SERS to address the two great shortcomings ofconventional Raman spectroscopy: low sensitivity and interfer-ence from luminescence. These characteristics make SERSuniquely suited to the analysis of organic colourants, bothnatural and synthetic (for a detailed review of SERS applicationsin cultural heritage studies, see ref. 2).

Of the metals used, silver is by far the most popular, givingnanoparticles that show high affinity for organic dyes, andresulting in excellent analytical sensitivity. Silver nanoparticlescan be synthesized to show surface-enhancement over the blueto near infrared range of laser excitation wavelengths (488–785nm), which is what most Raman instruments in cultural heri-tage institutions employ.

Silver nanoparticles, in the form of silver colloids, can beprepared easily in the laboratory with safe chemical proce-dures.3 SERS measurements are normally carried out witha standard Ramanmicrospectrometer, on amicroscopic sampleimmersed in a drop of silver colloid or decorated witha colloidal paste (a thicker slurry made by concentrating a silvercolloid by centrifugation) as shown in Fig. 1.

Procedure

SERS is classied as a micro-destructive technique: it can beperformed on a micro-sample taken from the object (usuallya fraction of a millimetre across), or in situ, i.e., on the objectitself. In the latter case, a drop of the metal colloid (usually 1mm across or less) is deposited on the surface to be analysed.

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Fig. 1 SERS analysis in practice: from top left, going left to right andtop to bottom: sampling a coloured paper fibre with a tungsten nee-dle; the sample; sample treatment with hydrofluoric acid (HF) to breakthe metal-dye molecule bond in the lake pigment; the same aftertreatment (scale in micrometres); silver colloid added to the sample;the colloid begins to aggregate following adsorption of the dye ontothe nanoparticles; the Raman microscope is focused into the colloiddrop (dark dots are large nanoparticles aggregates); the 488 nm laserbeam is focused into the drop; the SERS spectrum of eosin.

Fig. 2 (A) Fragment of a quiver; accession no. 28.3.5; Middle Kingdom,ca. 2124–1981 BC (H 11 cm; W 13 cm). MMA 1911–1912, TombMMA830, Thebes, el-Khokha, Upper Egypt; Rogers Fund, 1928. (B)Polarized reflected light photograph of sample removed from redpainted area before HF treatment (scale bar: 20 mm). (C) SERS spec-trum of sample from Middle Kingdom leather quiver. Solid line,spectrum of sample from red painted area; dashed line: spectrum ofa madder lake reference. The spectra were transposed vertically forclarity.

AMC Technical Briefs Analytical Methods

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When performing SERS on a sample, some authors havefound it advantageous to treat the sample with an acid tohydrolyse lake pigments or generally break the bond betweendye and its substrate. This procedure allows the dye mole-cules more easily to diffuse into the colloid drop and adsorbon the surface of the nanoparticles. In a recent developmentof the technique, a gelatine nanocomposite is prepared andput on the spot to analyse; aer the analysis, the gelatine canbe peeled off and removed without leaving either a markvisible to the naked eye, or traces of the metal nanoparticles.However, the gelatine is slightly wet and this may be an issuewith water-soluble pigments and dyes or on water-sensitivesubstrates.

What is SERS good for?

SERS has been used extensively in museum laboratories for theanalysis of a variety of cultural heritage objects spanning a largegeographical and chronological range and covering all types ofmaterials, including textiles, prints, drawings, paintings, andpolychrome sculpture.

The bulk of the published examples concerns red anthra-quinone dyes such as carmine and madder. For example, SERSwas used to document the oldest known so far example of a lakepigment made from madder, in an Egyptian leather fragmentdating to the Middle Kingdom (Fig. 2), as well as the rstoccurrence of lac dye in western European art, in a Romanesquesculpture from Southern France.3

SERS is equally suitable to the analysis of textile bres,watercolor and tempera paints, and lake pigments in oil (for anexample of the latter, see ref. 4).

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SERS has been applied, in addition to anthraquinones suchas madder, cochineal, kermes, and lac dye, to all major naturalcolourants used as textile dyes and lake pigments, and to a goodnumber of synthetic ones (see for instance ref. 5–7). A summarylist would include weld, fustic, berberine, logwood, safflower,brazilwood, orchil, shikon, methyl violet, magenta, mauveine,and eosin.

Advantages and limitations

The chief advantage of SERS is its extremely high sensitivity,which makes it possible to identify target molecules frombres or paint samples as small as 20 micrometres. This isparticularly important given the relatively poor performance ofnon-invasive methods with the majority of organic dyes.Before the introduction of SERS in the cultural heritage eld,denitive identication of an organic dye was a task generallyaccomplished using high-performance liquid chromatography(HPLC). While a few examples of the application of HPLC topaintings exist, the majority of HPLC work concerns textilessuch as carpets and tapestries, from which a fewmillimetres oftextile bres can be obtained. SERS has enabled researchers inmuseums to work as routine on paintings (Fig. 3) and otherobjects from which sampling is more limited.

The key limitation of SERS is particularly evident when thetechnique is compared to HPLC. Unlike chromatography, whereindividual components of a mixture are separated and thenanalysed with a spectroscopic method, the spectral response inSERS is the aggregate response of all components. To compli-cate the picture, some dyes show a greater affinity for adsorp-tion on silver than even closely related dyes, making it difficultto resolve dye mixtures.8

Anal. Methods, 2017, 9, 4338–4340 | 4339

Fig. 3 SERS spectra from ‘The Card Players’, Paul Cezanne, MMA61.101.1. (a) Reference madder lake upon HF treatment, (b) sample onAg colloid upon HF treatment; (c) sample on 5� Ag colloid withouthydrolysis (peaks marked with * are due to the colloid itself). (1) Thepainting. (2) The sample under the stereomicroscope.

Analytical Methods AMC Technical Briefs

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Analysis of spectra

The analysis of SERS spectra is accomplished by comparison withreference spectra. Spectral databases have been assembled byvarious groups.5,6 Automated or semi-automated digital spectrallibrary search methods have been evaluated, demonstrating theirreliability.9 The limitation highlighted in the previous paragraphconcerning analysis of mixtures should, however, be kept inmind. Finally, assignment of spectral peaks to specic vibrationsis complicated by surface-enhancement and its specic selectionrules. This task generally requires computational ab initiomethods to simulate the Raman spectrum.

Marco Leona (The Metropolitan Museum of Art, New York).

This Technical Brief was prepared by the Heritage ScienceSubcommittee and approved by the Analytical MethodsCommittee on 29/06/17.

4340 | Anal. Methods, 2017, 9, 4338–4340

References

1 B. Guineau and V. Guichard, ICOM Committee forConservation: 8th Triennial Meeting, Preprints, The GettyConservation Institute, Marina del Rey, CA, 1987, Vol. 2, p.659.

2 F. Pozzi and M. Leona, Surface-enhanced Ramanspectroscopy in art and archaeology, J. Raman Spectrosc.,2015, 47, 67–77.

3 M. Leona, Proc. Natl. Acad. Sci. U. S. A., 2009, 106, 14757.4 F. Pozzi, K. J. van den Berg, I. Fiedler and F. Casadio, A

systematic analysis of red lake pigments in Frenchimpressionist and post-impressionist paintings by surface-enhanced Raman spectroscopy (SERS), J. Raman Spectrosc.,2014, 45, 1119–1126.

5 M. Leona, J. Stenger and E. Ferloni, Application of surface-enhanced Raman scattering techniques to the ultra-sensitive analysis of natural dyes in works of art, J. RamanSpectrosc., 2006, 37, 981–992.

6 S. Bruni, V. Guglielmi and F. Pozzi, Historical organic dyes:a surface-enhanced Raman scattering (SERS) spectraldatabase on Ag Lee–Meisel colloids aggregated by NaClO4,J. Raman Spectrosc., 2011, 42, 1267–1281, DOI: 10.1002/jrs.2872.

7 I. Geiman, M. Leona and J. R. Lombardi, Application ofRaman spectroscopy and SERS to the analysis of syntheticdyes found in ballpoint inks, J. Forensic Sci., 2009, 54, 947–952.

8 F. Pozzi, S. Zaleski, F. Casadio and R. P. Van Duyne, SERSdiscrimination of closely related molecules: a systematicstudy of natural red dyes in binary mixtures, J. Phys. Chem.C, 2016, 120, 21017–21026.

9 F. Pozzi, S. Porcinai, J. R. Lombardi and M. Leona,Statistical methods and library search approaches for fastand reliable identication of dyes using surface-enhancedRaman spectroscopy (SERS), Anal. Methods, 2013, 5, 4205–4212.

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